This invention relates to a method of etching a surface by exposing it to a RF discharge plasma containing a chemical species that reacts with the surface to form a gaseous product. The plasma etching method of this invention is particularly useful in the manufacture of integrated circuit chips and related semiconductor devices.
Semiconductor chips are typically manufactured by subjecting a silicon wafer to a predetermined sequence of surface treatment operations to form the desired electrically operative features. At some stages, it is desired to remove material from selected areas of the wafer surface. One removal process calls for exposing the wafer surface to a RF discharge plasma containing reactive etching species. The plasma is generated by applying a radio frequency (RF) signal to a low pressure gas. A plasma generated in a suitable gas, such as carbon tetrafluoride, creates chemical species that collide with the wafer surface and react with the exposed material. The reaction forms gaseous products, most notably silicon fluoride, that diffuse into the atmosphere. While the reaction mechanism is not well understood, it is believed that fluorine atoms and other fluorine-containing radicals play a predominate role. This is in contrast to sputter etching wherein a plasma discharged in an inert gas such as argon produces excited ions that violently impact the surface and physically knock material away.
It is known that a RF discharge plasma in carbon tetrafluoride gas etches silicon and also silicon compounds typically used as semiconductor overlayers, such as silicon dioxide SiO.sub.2, silicon nitride Si.sub.3 N.sub.4 and polysilicon. A given plasma etches these materials at different rates. Typically, EQU E.sub.Si.sbsb.3 .sub.N.sbsb.4 &gt;E.sub.Si &gt;E.sub.SiO.sbsb.2
where E.sub.x represents the etch rate of material X.
Whatever material is being etched, faster etch rates are generally desired to reduce processing time and power. Adjusting the discharge to increase the etch rate of a particular material is frequently not satisfactory. In some instances, it may be desired to decrease the etching of a particular material by a predetermined plasma. Therefore, it is an object of this invention to provide a method capable of selectively increasing or decreasing the etch rate of a desired material exposed to a RF plasma without perceptibly altering the discharge power, the gas pressure or other plasma parameters.
It has also been heretofore difficult to simultaneously etch two wafers exposing different materials having different etch rates. For a given processing time, one wafer was overetched or the other was not completely etched. Likewise it has been a problem to etch different materials on the same wafer. For example, when opening a window in the SiO.sub.2 film on a silicon base, it is desired to minimize the attack upon the silicon. But the etch rate for silicon is typically much higher than for silicon dioxide and so the plasma roughens or pits the freshly exposed silicon. In short, better control over the relative etch rates of different materials exposed to a predetermined RF discharge plasma would provide additional processing flexibility and would permit higher quality semiconductor devices and circuits to be produced.
Therefore, it is an object of this invention to provide a method for better controlling the etch rates of two or more materials exposed to a predetermined RF discharge plasma containing reactive etching species. This is accomplished without necessarily changing the RF signal or the nature of the gas. The improved etch control of this invention can be exerted in a selected region of the plasma or during selected processing times without interrupting or affecting the overall plasma discharge. It is a more specific object of this invention to provide such a method for selectively adjusting the relative etch rates of two or more silicon-containing materials subjected to a single predetermined RF discharge plasma containing reactive etching species, which method is selectively exercisable independent of the plasma parameters to produce an improved etch pattern for semiconductor wafer manufacture.
Another problem encountered in plasma etching semiconductor wafers is that the etch rates are generally not uniform. For example, etch rates are usually faster about the circumference of the wafer than near the center. Also when processing a plurality of silicon wafers concurrently, it has been found that etch rates may vary from wafer to wafer depending upon their position in the plasma apparatus. It is therefore a further object of this invention to provide a method for improving the uniformity of etch rates of a desired material subjected to a predetermined RF discharge plasma containing a reactive etching species across a wafer surface and among the surfaces of a plurality of wafers.